Method and system for detecting and treating junctional rhythms

ABSTRACT

A method and an apparatus for treating cardiac arrhythmias are provided. An interval between first and second consecutive beats of a heart, having first and second chamber types, is determined. The heart is paced at a first rate if the first beat is from the first chamber type and the second beat is from the second chamber type and the interval is less than a predetermined amount of time or if the first and second beats are both from the second chamber type. The heart is paced at a second rate if the first beat is from the first chamber type and the second beat is from the second chamber type and the interval is more than the predetermined amount of time.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/565,188, filed Nov. 30, 2006 entitled “METHOD AND SYSTEM FORDETECTING AND TREATING JUNCTIONAL RHYTHMS”, herein incorporated byreference in its entirety.

This application claims the benefit of U.S. Provisional Application No.60/740,947, filed Nov. 30, 2005, the disclosure of which is incorporatedin its entirety.

TECHNICAL FIELD

The present invention generally relates to cardiac pacing therapymethods and systems, and more particularly relates to methods andsystems for detecting junctional rhythms and delivering pacing therapiesto treat junctional rhythms.

BACKGROUND

In recent years, the use of implantable cardiac devices, such aspacemakers and defibrillators, has become increasingly common. Suchdevices are now used not only to treat and terminate cardiacarrhythmias, but also to provide valuable information to the clinicianregarding the implanted device and its interactions with the patient.

Normal heart rhythm is termed normal sinus rhythm. The initiatingimpulse for a heart beat normally begins in an area high (towards thepatient's head) in the right atrium called the sinus node. Electricalpropagation spreads to the left atrium and down the right atriumsimultaneously. Mechanical contraction follows electrical activation bya delay in the order of hundreds of milliseconds. At the base of theatria, the electrical signals converge on the atrio-ventricular (AV)node. The AV node conducts electrical impulses slowly from the atria tothe ventricles, which provides blood from the atrial contraction time tofill the ventricles. When this relationship is appropriate and normal,as in normal sinus rhythm, the heart is said to be operating withatrio-ventricular (or AV) synchrony.

One type of arrhythmia known as a “junctional rhythm” results in a lossof proper atrio-ventricular synchrony. The term “junction” refers to the“AV junction”, also known as the “AV node”, and such rhythms aresometimes called nodal rhythms. During a junctional rhythm, theinitiating impulse originates in the AV junction. Subsequently, theatria and ventricles are activated electrically and contractmechanically at nearly the same time, since retrograde conduction of theimpulse from the AV node to the atria and antegrade conduction from theAV node to the ventricles takes about the same time. Near simultaneouscontraction of the atria and ventricles does not allow transport ofblood from the atria to the ventricles. The impact of this impropertiming includes higher pressures in the atria, retrograde flow of bloodinto the veins that feed blood to the heart, and lack of ventricularfilling. The latter results in a compromised hemodynamic status.

The AV node beats spontaneously at a rate faster than the sinus node orany other part of the heart in order to establish and maintain ajunction rhythm. Generally, the portion of the heart which beats mostrapidly establishes and controls the cardiac rhythm. If the heart rateis fast, it is called tachycardia. While tachycardia was initiallydescribed as greater than 100 beats per minute, it has come to be usedin a more relative sense. A heart rate which is faster than appropriatefor a given physiologic condition of a patient is now generallyconsidered tachycardia although the rate may be less than the classicalthreshold of 100 beats per minute. Thus, a junctional rhythm issometimes called junctional tachycardia.

The source of the junctional beat may also be referred to as an ectopicbeat. A beat which originates from a location other than that whichwould be normal is said to be “ectopic”. As example, an ectopic atrialbeat is one which does not originate from the sinus node. Beats whichoriginate from the ventricles are ventricular ectopic beats. Junctionalrhythms may also be called junctional ectopic tachycardia andabbreviated JET.

Symptoms associated with junctional rhythms are a result of the loss ofAV synchrony resulting from aberrations of the normal conductionsequence of the heart. These symptoms may include shortness of breath,choking sensation, chest pain, fatigue, anxiety, dizziness, andconfusion, all of which are generally considered to be signs ofdecreased cardiac output. In some instances, pulsations may manifest inthe neck and abdomen of the patient due to the contraction of the atriaon closed AV valves, which forces blood back into the venous system.Hypotension, pallor, cool extremities, diaphoresis, reduced urineoutput, jugular venous distension, ascites, and hepatojugular reflex canalso be associated with junctional tachycardia.

Junctional tachycardia occurs frequently following surgical proceduresto correct congenital heart defects (often children) and repair heartvalves. These rhythms may occur because the patient's sinus node is slowor their AV node is fast. On an ambulatory basis, some patientsexperience junctional rhythms. Some are brief, infrequent episodes forwhich patients are asymptomatic, while others produce symptoms and/orare persistent. Since episodes occur infrequently, it is unlikely thatsuch an episode will occur during examination of patients who haveimplanted devices such as a pacemaker or defibrillator. Junctionalrhythms typically are not recognized during routine ambulatory ECGmonitoring of patients with devices, as recognition of this arrhythmiais not reported by such systems. However, ambulatory recording withspecial monitors that incorporate telemetry from the implanted rhythmmanagement device combined with algorithms designed to detect junctionalrhythms has demonstrated that junction beats and junctional rhythmsexist with a far higher frequency than previously understood in typicalpacemaker or defibrillator (ICD) patient populations.

Dual chamber pacemakers and ICDs typically pace and sense the rightatrium and right ventricle. Cardiac resynchronization therapy (CRT)devices pace and sense the right atrium, right ventricle and the leftventricle. One feature of such devices is the maintenance ofatrio-ventricular synchrony. CRT devices restore and maintain interventricular synchrony, between left and right ventricles. These devicesprevent bradycardia by maintaining a suitable atrial rate with atrialpacing and (A-V) ventricular synchrony with ventricular pacing. DuringAV block, ventricular pacing synchronized to spontaneous atrial beats oratrial pacing maintains proper A-V timing. Junctional rhythms, however,often render pacemakers inhibited.

If junctional rhythms produce atrial beats closely followed byventricular beats, dual chamber pacemakers may sense the atrial beat butthe closely following sensed ventricular beat causes the pacemaker topresume the rhythm is normal sinus rhythm, and no pacing will occur. Ifjunctional rhythms produce atrial beats closely following ventricularbeats, the action is essentially the same. Sensed ventricular beatscause the timing of the pacemaker to be reset and resume a new timingcycle for subsequent beats. Sequences of consecutive ventricular beatsare generally recognized by pacemakers as premature ventricularcontractions (PVCs) and recorded as such for later interrogation byclinicians.

Thus, implanted pacemakers are generally ineffective at treatingjunctional rhythms and physicians are presented with few if any clues inthe patient's record as to the possible occurrence of such rhythms. Ifpatients experience infrequent symptoms, their presentation duringfollow-up is with few if any clues about the possible occurrence ofjunctional rhythms as the explanation. For the patient in whomjunctional rhythms are recognized, therapeutic options are quitelimited. Raising the pacemaker rate allows the atrium to become fasterthan the junctional rhythm, but this option is nearly alwaysunsatisfactory. While raising the rate of pacing when the patientpresents in clinic for follow-up may allow restoration of AV synchronyand control of heart rate with the pacemaker at that time, activities ofdaily living will frequently cause the junctional rhythm to exceed theprogrammed rate of the pacemaker and loss of AV synchrony will againrecur. Reprogramming the pacemaker to a higher rate that will overcomethe junctional rhythm completely for all time is often at a rate whichcan not be comfortably or safely sustained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is an isometric view of a cardiac treatment system, including animplantable cardiac device and a lead set;

FIG. 2 is a block diagram of a system within the implantable cardiacdevice illustrated in FIG. 1;

FIG. 3 is a flow chart of a method for detecting and treating junctionalrhythms;

FIG. 4 is a flow chart of a method for detecting junctional rhythms withthe introduction of atrial pacing;

FIG. 5 is a flow chart of a method of detecting junctional rhythmswithout the use of pacing; and

FIG. 6 is a graph illustrating the relationship between pacing rate andthe atrio-ventricular interval of a heart during a method for treatingjunctional rhythms according to one embodiment of the present invention.

FIGS. 7 through 9C relate to another embodiment of the invention.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary, or the following detailed description. Itshould also be understood that FIGS. 1-6 are merely illustrative and maynot be drawn to scale.

FIG. 1 to FIG. 6 illustrate a method and system for detecting andtreating junctional rhythms. In one embodiment, after a junction rhythmis detected, the atrioventricular (AV) interval of the heart ismonitored and the pacing rate is adjusted based on the detected AVinterval. If the AV interval falls below a predetermined threshold(e.g., 125 mS), the pacing rate is increased. If the AV interval risesabove (or greater than or equal to) the predetermined threshold, thepacing rate is decreased. Even if during pacing, the detected AVinterval indicates that the heart is in proper AV synchrony, the pacingrate may still be reduced to test whether or not the heart is stillexperiencing a junctional rhythm. If the heart is still experiencing ajunctional rhythm and the pacing rate is reduced to a rate that is belowthe junctional rhythm rate, the AV interval will decrease. Once the AVinterval decreases below the predetermined threshold, the pacing rate isagain increased until proper AV synchrony is achieved. This is repeatedperiodically to accommodate varying conditions of the patient and thejunctional rhythm. Thus, the method and system pace the heart at thelowest possible rate to maintain proper AV synchrony.

FIG. 1 illustrates a cardiac treatment system 10 according to oneembodiment of the present invention. The system 10 includes animplantable cardiac device, or implantable cardioverter defibrillator(ICD), 12 and a lead set 14, which are connected to a heart 16. The ICD12 includes a housing 18, a connector block 20, and lead connectorassemblies 22. The ICD 12 may be, for example, a pacemaker,cardioverter, and/or defibrillator, as is commonly understood in theart. Although not illustrated in detail, an uninsulated portion of thehousing 18 may function as an electrode to defibrillate either the atriaor ventricles of the heart. This uninsulated portion may also be used asan electrode for sensing, pacing and other electrical measurements suchas by measuring impedance for a variety of diagnostic functions.

The lead set 14 includes a ventricular lead 24, an atrial/superior venacava (SVC) lead 26, and a coronary sinus lead 28. As will be appreciatedby one skilled in the art, the ICD 12 and the lead set 14 illustrated inFIG. 1 are in the form of a “transvenous” ICD system, as the leads 24,26, and 28 have been inserted into the heart 16 through the venoussystem. The ventricular lead 24 includes an elongated insulated leadbody 30 that carries three concentric coiled conductors, separated fromone another by tubular insulated sheaths. The ventricular lead 24 alsoincludes a ring electrode 32, an elongated coil electrode 38, and anextendable helix electrode 34, mounted retractably within an insulatedbody 36. Although not specifically illustrated, electrodes 32, 34 and 38are each coupled to one of the coiled conductors within lead body 30 andcan be used for both ventricular pacing and sensing of ventriculardepolarizations.

The atrial/SVC lead 26 includes an elongated insulated lead body 40,similar to lead body 30, carrying three concentric coiled conductors,separated from one another by tubular insulated sheaths. The atrial/SVClead 26 also includes a ring electrode 44 and an extendable helixelectrode 42, mounted retractably within an insulated body 46. Thedistal end of lead body 40 is preformed to retain a J-shape at thedistal end. The electrodes 42 and 44 are each coupled to one of thecoiled conductors within lead body 40 and are employed for atrial pacingand sensing of atrial depolarizations. An elongated coil electrode 48 isprovided proximal to ring electrode 32 and coupled to the thirdconductor within lead body 40.

Lead 28, lead body 50, and hidden lead body profile 52 at the distal endthereof are placed through the coronary sinus, venous drainage from theheart's circulation, and into a further tributary such as the greatcardiac vein. The distal end of lead body 50 includes an electrode, notshown in FIG. 1, for use in pacing and sensing the left ventricle.

The lead connector assemblies 22 include a right ventricular leadconnector 54, an atrial/SVC lead connector 56, and a left ventricularlead connector 58. The right ventricular lead connector 54 is bifurcatedand carries two electrical connectors. The atrial/SVC lead connector 56is bifurcated but carries three electrical connectors, each coupled to arespective conductor within the ventricular lead connector 54. The leadconnector 58 is connected to the single conductor within the lead body50. The leads 24, 26, and 28 through the lead connectors 54, 56 and, 58pass into the connector block 20. Electrical and mechanical connectionsare made in connector block 20 such that each conductor is electricallyrouted into the housing 18.

FIG. 2 is a functional block diagram illustrating a system capable ofperforming the methods of detecting and treating cardiac arrhythmias,according to one embodiment of the present invention. The system 60 maybe implemented within the ICD 12 of FIG. 1 and may take the form of animplantable device that integrates variouspacemaker/cardioverter/defibrillator functions. FIGS. 1 and 2 areexemplary of the type of device in which the invention may be embodied,as the invention may be practiced in a wide variety of deviceimplementations, including devices providing therapies for treatingatrial arrhythmias, ventricular arrhythmias, and junctional rhythms. Inaddition, the invention may be practiced in pacemakers that do notprovide cardioversion or defibrillation, as well as devices that deliverdifferent antiarrhythmic therapies such as nerve stimulation or drugadministration. As will be appreciated by one skilled in the art, thesystem 60 illustrated in FIG. 2 may be appropriate for pacing andsensing two chambers of a heart. A system suitable for pacing andsensing three chambers of the heart, such as in cardiacresynchronization therapy (CRT), would include, in addition to thecomponents described below, an additional sense amplifier, blankingcontrol and signals for the third sense amplifier, and an LV pacingoutput circuit.

In the example illustrated in FIG. 2, electrodes 32, 34, 38, 42, 44, and48 represent the electrodes designated by similar reference numeralsshown in FIG. 1, as well as the electrode on the distal end of the leadbody 50. Likewise, electrode 61 represents the uninsulated portion ofthe housing 18 of device 10, as illustrated in FIG. 1, which mayfunction as a defibrillation, sensing, pacing, and measurementelectrode. Referring again to FIG. 2, electrodes 61, 38, and 48 arecoupled to a high voltage output circuit 62 (e.g., pulse generator).Electrodes 32 and 34 are coupled to an R-wave amplifier 64 for the rightventricle, which preferably takes the form of an automatic gaincontrolled amplifier providing an adjustable sensing threshold as afunction of the measured R-wave signal. A signal is generated on anR-out line 66 whenever the signal sensed between electrodes 32 and 34exceeds the present sensing threshold.

Electrodes 42 and 44 are coupled to a P-wave amplifier 68, which alsomay take the form of an automatic gain controlled amplifier providing anadjustable sensing threshold as a function of the measured P-waveamplitude. A signal is generated on a P-out line 70 when the signalsensed between electrodes 42 and 44 exceeds the sensing threshold. Aswitch matrix 72 selects which of the available electrodes are coupledto a wide band amplifier 74 for use in digital signal analysis. Theselection of which of the electrodes are to be operated is controlled bya controller, which may take the form of a microprocessor (μP) 76. TheμP 76 controls selection of the electrodes via the switch matrix 72through a data/address bus 78. Signals from the electrodes selected forcoupling to the wide band amplifier 74 are provided to a multiplexer(MUX) 80 and thereafter converted to multi-bit digital signals by ananalog-to-digital (A/D) converter 82, for storage in a random accessmemory (RAM) 84 under control of a direct memory access (DMA) circuit86.

The μP 76 may preferably employ digital signal analysis techniques tocharacterize the digitized signals stored in the RAM 84 to recognize andclassify the heart rhythm (e.g., normal sinus rhythm, arrhythmia, etc.)using any of a variety of known signal processing methods. Inparticular, the μP 76 may implement a detector that monitors the cyclelength and regularity of the heart rhythm to identify and classify anarrhythmia. The remainder of the circuitry illustrated in FIG. 2, suchas pacer timing and control circuitry 88, output circuits (i.e., A pacecircuit 90 and V pace circuit 92), cardioversion/defibrillation controlcircuitry (CV/DEFIB Control) 94, and high voltage charging circuit 96,is dedicated to the provision of cardiac pacing, cardioversion, anddefibrillation therapies. The μP 76 is programmed to control thecircuitry of FIG. 2 to detect and classify different heart rhythms, makeand record measurements, and deliver pacing therapies.

Although not specifically illustrated, it should be understood that thesystem 60, either as hardware or instructions stored, for example,within the μP 76, the RAM 84, and/or pacer timing and control circuitry88, also includes various predetermined AV intervals, such as lower andupper AV interval limits, rate-adaptive AV intervals, and separate AVintervals associated with atrial pacing and atrial sensing. These AVintervals are adjustable by, and information stored in the ICD 12 isaccessible to, a user (e.g., physician, nurse, or technician) of thesystem 60 illustrated in FIG. 1 through a telemetry system. Alsoincluded are various V-V intervals, or interventricular intervals (i.e.,the time intervals between successive, but not necessarily consecutive,premature ventricular beats), such as the time between sensing on aright ventricular lead and pacing on the left ventricular lead, sensingon a left ventricular lead and pacing on the right ventricular lead,pacing on the left ventricular lead followed by pacing on the rightventricular lead, and/or pacing on the right ventricular lead followedby pacing on the left ventricular lead. The system 60 also includes(e.g., stored on RAM 84) instructions for carrying out the methods andprocesses described below.

In use, referring again to FIG. 1, after the treatment system 10 hasbeen surgically implanted into the patient, the system 60 may applyanti-bradycardia pacing, anti-tachyarrhythmia pacing or other pacingtherapies to the heart 12, or may simply monitor the activity of theheart 12 (i.e., detection mode).

FIG. 3 illustrates a method 100 for detecting and treating a junctionalrhythm according to one embodiment of the present invention. The processbegins at step 102 with the ICD 12 being implanted into a patient andcoupled to the heart of the patient, as described above. Although notspecifically illustrated, the ICD 12 may begin pacing the heart with oneof the pacing methods described above before the system 60 implementsthe method 100.

At step 104, the ICD 12 “suspects,” or makes a preliminary detection of,a junctional rhythm in the heart of the patient. The ICD 12 may make thepreliminary detection of the junctional rhythm through the electrodesby, for example, either detecting an atrial depolarization just before aventricular depolarization or detecting ventricular depolarizations inthe absence of atrial depolarizations. That is, the ICD 12 suspects ajunctional rhythm if two consecutive beats of detected, one being anatrial beat and the other being a ventricular beat, and the intervalbetween the two beats is less than the predetermined threshold describedabove. The ICD 12 also suspects a junctional rhythm if the twoconsecutive beats are both ventricular beats, with no atrial beatoccurring in the interval between.

After the ICD 12 has made the preliminary detection of the junctionalrhythm, the ICD 12 may then attempt to control the ventricular rateusing atrial pacing. At step 106, the ICD first determines the RRinterval (i.e., time between ventricular beats). Then at step 108, it isdetermined whether or not the RR interval (i.e., the ventricular rate)is stable. The ventricular rate may be deemed to be stable if the RRinterval stays within a predetermined threshold, which may be based onthe first several RR intervals that are detected. If the ventricularrate is not stable, the process continues to step 110 where no attemptis made to treat the arrhythmia.

If it is determined that the RR interval is stable, the processcontinues to step 112, at which point an atrial pace is scheduled andinserted at an appropriate interval before the next ventricular beat. Inone embodiment, the atrial pace is inserted between 100 and 200 msbefore the next anticipated ventricular beat. The inserted ventricularbeat may be the beginning of an atrial pacing session or a rescheduledatrial pace of a pacing therapy that had already started. At step 114,it is again determined whether or not the ventricular rate is stable(i.e., the atrial paces are followed by a constant ventricular beat). Ifthe ventricular rate is not stable, the process again moves to step 110and no attempt is made it treat the arrhythmia.

If the ventricular rate remains stable after atrial pacing has begun, atstep 116, the atrial pacing rate is changed (from a first test rate to asecond test rate), and at step 118, the ventricular rate is againchecked to see if the RR interval has changed according to the newatrial pacing rate. In one embodiment, the atrial pacing rate isincreased by approximately 5 bpm. If the R-wave follows the atrialstimulus provided by the atrial pacing, the ventricular rate will alsoincrease similarly to the atrial pacing rate. If the ventricular ratefollows the change in atrial pacing, it is assumed that the heart is notexperiencing a junctional rhythm and the process returns to thebeginning. However, if the ventricular rate does not follow the changein atrial pacing, the process continues to step 120 at which point ajunctional rhythm treatment therapy is delivered, as will be describedbelow.

FIG. 4 illustrates a method 122, or algorithm, for treating junctionalrhythms according to one embodiment of the present invention. At step124, the process begins with the determination having been made that theheart is experiencing a junctional rhythm. At step 126, in a mannersimilar to that described above, the ICD 12 determines the intervalbetween two consecutive beats of the heart. As described before, one ofthe beats may be an atrial beat and the other may be a ventricular beat,or both beats may be ventricular beats.

At step 128, it is determined whether or not both beats are ventricularbeats. If both beats are ventricular, the process continues to step 130at which point the atrial pacing rate is increased from an initial rateor initiated if pacing had not yet commenced. The process then returnsto step 126 where the interval between the next two consecutive beats isdetermined. If one of beats is an atrial beat and the other is aventricular beat, from step 128 the process continues to step 132.

At step 132, it is determined whether or not the AV interval (i.e., thetime between the atrial beat and the ventricular beat) is within thenormal AV interval limits that are stored in the memory of the ICD. Ifthe AV interval is within the AV interval limits, the process returnsagain returns to step 126. In one embodiment, the AV interval limitsrange between 100 and 150 ms. That is, if the AV interval is between 100and 150 ms, the heart is determined to be in normal AV synchrony. Inanother embodiment, the AV interval limits cover only a single AVinterval (i.e., a target AV interval, such as 120 ms). In such anembodiment, if the sensed AV interval is not exactly 120 ms, the AVinterval is determined not to be within the AV interval limits.

If the AV interval is not within the AV interval limits, the processcontinues to step 134. At step 134, it is determined whether or not thesensed AV interval is greater than the AV interval limits. If the AVinterval is greater than the AV interval limits, or the target AVinterval, the process continues to step 136 at which point the atrialpacing rate is decreased. If the sensed AV interval is not greater thanthe AV interval limits, the process continues to step 138. At step 138,it is determined whether or not the AV interval is less than the AVinterval limits. If the AV interval is not less than the AV intervallimits, the process return to step 126. If the sensed AV interval isless than the AV interval limits, the process returns to step 130, andthe atrial pacing rate is increased.

Thus, the method 122 essentially slows the atrial pacing rate to providea measure of AV conduction time. Once the atrial pacing rate has slowedbelow the rate of the junctional rhythm, the time between the atrialstimulus (or beat) and the detection of the ventricular beat willdecease. If the atrial pacing rate becomes to slow, the junctionalrhythm will dominate and the ventricular beat from the junction rhythmmay precede the atrial pace, at which point the atrial pacing rate isincreased. In this way, the method 122 continuously searches for theproper atrial pacing rate to provide proper AV synchrony.

Although not specifically illustrated in FIG. 4, the rate at which theatrial pacing is increased may differ depending on whether the twoconsecutive beats are both ventricular, as determined at step 128, orthe sensed AV interval is less than the AV interval limits. In oneembodiment, the atrial pacing rate is increased by 1 bpm per beat if thesensed AV interval is less than the AV interval limits. The atrialpacing rate may be increased more quickly (e.g., 2 bpm per beat) if bothbeats are determined to be ventricular beats.

Referring again to FIG. 2, during operation, various types of data maybe stored by the system 60 (e.g., in the microprocessor 76, RAM 84, DMA86, or other memory). The system 60 records and stores informationrelative to heart rhythms and interaction of the ICD 12 with the heart16. For example, specific information is maintained regarding each beatand whether it was paced or sensed in the atrium and, similarly, pacedor sensed in the ventricle. The system 60 also recognizes and storestime-stamped episodes along with their duration, the rate at which theyoccurred, and the activity level of the patient (i.e. sensor derivedrate for rate response). Such information will enable a clinician torecognize occurrences of junctional rhythms in the patient andappropriately adjust any settings within the ICD 12, as well asprescribe other treatments to prevent future occurrences. As will beappreciated by one skilled in the art, the information stored within thesystem 60 may be accessed by the clinician through a telemetry system.

One advantage of the method and apparatus described above is thatjunctional rhythms may be detected and treated by an ICD. Anotheradvantage is that during an occurrence of a junction rhythm, proper AVsynchrony is restored to the heart while pacing the heart at the slowestrate possible. A further advantage is that because the slowest possiblepacing rate which will retain proper AV synchrony is constantly besought, if the heart becomes capable to operate with a normal sinusrhythm, the pacing rate will drop below the sinus rate and the pacingwill cease.

Other embodiments may utilize other types of ICDs besides transvenous,such as epicardial and subcutaneous ICDs. The methods and algorithmsdescribed above may be implemented separately, or in combination as asingle method to detect and treat junctional rhythms.

FIGS. 7 through 9C relate to another embodiment of the invention.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A method for treating junctional rhythms comprising: determining aninterval between first and second consecutive beats of a heart havingfirst and second chamber types; pacing the heart at a first rate if thefirst beat is from the first chamber type and the second beat is fromthe second chamber type and the interval is less than a predeterminedamount of time or if the first and second beats are both from the secondchamber type; and pacing the heart at a second rate if the first beat isfrom the first chamber type and the second beat is from the secondchamber type and the interval is more than the predetermined amount oftime.
 2. The method of claim 1, wherein the first pacing rate is greaterthan the second pacing rate.
 3. The method of claim 2, wherein thepredetermined amount of time is a range between a lower interval limitand an upper interval limit.
 4. The method of claim 3, furthercomprising pacing the heart at an initial pacing rate and wherein saidpacing at the first rate comprises increasing the initial pacing rate ata first pacing increase rate if the first beat is from the first chambertype and the second beat is from the second chamber type and theinterval is less than the lower interval limit and increasing theinitial pacing rate at a second pacing increase rate if the first andsecond beats are both from the second chamber type, the second pacingincrease rate being greater than the first pacing increase rate.
 5. Themethod of claim 4, wherein the first chamber type is an atrium and thesecond chamber type is a ventricle.
 6. The method of claim 5, whereinsaid pacing and said determining are performed with an implantablecardioverter defibrillator (ICD) implanted subcutaneously in thepatient.
 7. The method of claim 6, further comprising repeating saidpacing and said determining for a plurality of sets of consecutivebeats.
 8. The method of claim 7, wherein the lower interval limit isapproximately 100 ms and the upper interval limit is approximately 150ms.
 9. The method of claim 8, wherein said pacing is atrial pacing andfurther comprising determining a ventricular rate of the heart andpacing the heart at a test pacing rate, said pacing at the test pacingrate comprising inserting an atrial pace a predetermined AV intervalbefore a next ventricular beat.
 10. The method of claim 9, furthercomprising: changing the test pacing rate; and determining theventricular rate after said change in the test pacing rate.
 11. Animplantable cardiac device comprising: a housing; a sense amplifierwithin the housing and responsive to depolarizations of at least onechamber type of a heart, the heart having first and second chambertypes; a pulse generator within the housing to produce cardiacstimulation pulses to the heart; and a controller within the housing andcoupled to the sense amplifier and the pulse generator, the controllerconfigured to: determine an interval between first and secondconsecutive beats of a heart having first and second chamber types; pacethe heart at a first rate if the first beat is from the first chambertype and the second beat is from the second chamber type and theinterval is less than a predetermined amount of time or if the first andsecond beats are both from the second chamber type; and pace the heartat a second rate if the first beat is from the first chamber type andthe second beat is from the second chamber type and the interval is morethan the predetermined amount of time.
 12. The device of claim 11,wherein the first pacing rate is greater than the second pacing rate andthe predetermined amount of time is a range between a lower intervallimit and an upper interval limit.
 13. The device of claim 12, whereinthe controller is further configured to pace the heart at an initialpacing rate and wherein said pacing at the first rate comprisesincreasing the initial pacing rate at a first pacing increase rate ifthe first beat is from the first chamber type and the second beat isfrom the second chamber type and the interval is less than the lowerinterval limit and increasing the initial pacing rate at a second pacingincrease rate if the first and second beats are both from the secondchamber type, the second pacing increase rate being greater than thefirst pacing increase rate.
 14. The device of claim 13, wherein thefirst chamber type is an atrium and the second chamber type is aventricle.
 15. The device of claim 14, further comprising a memorywithin the housing to record the intervals between the first and secondconsecutive beats and the occurrences of the first and second pacingrates.
 16. A method for treating cardiac arrhythmias comprising:determining an interval between first and second consecutive beats of aheart having an atrium and a ventricle; if the first beat is an atrialbeat and the second beat is a ventricular beat and the interval is lessthan a predetermined amount of time or if the first and second beats areboth ventricular beats, determining a ventricular rate for the heart;pacing the heart at a first test pacing rate, said pacing at the firsttest pacing rate comprising inserting an atrial pace a predetermined AVinterval before a next ventricular beat; pacing the heart at a secondtest pacing rate; re-determining the ventricular rate of the heart aftersaid pacing at the second test pacing rate; and delivering a junctionalrhythm therapy to the heart if the ventricular rate does not match thesecond test pacing rate.
 17. The method of claim 16, further comprisingaborting said delivery of the junction rhythm therapy if the ventricularrate is not stable.
 18. The method of claim 17, wherein said delivery ofthe junction rhythm therapy comprises: if the first beat is an atrialbeat and the second beat is a ventricular beat and the interval is lessthan a predetermined amount of time or if the first and second beats areboth ventricular beats, pacing the heart at a first rate; and if thefirst beat is an atrial beat and the second beat is a ventricular beatand the interval is more than the predetermined amount of time, pacingthe heart at a second rate.
 19. The method of claim 18, wherein thefirst pacing rate is greater than the second pacing rate and said pacingat the first rate comprises increasing the initial pacing rate at afirst pacing increase rate if the first beat is from the first chambertype and the second beat is from the second chamber type and theinterval is less than the lower interval limit and increasing theinitial pacing rate at a second pacing increase rate if the first andsecond beats are both from the second chamber type, the second pacingincrease rate being greater than the first pacing increase rate.
 20. Themethod of claim 19, wherein the predetermined amount of time is a rangebetween approximately 100 ms and 150 ms.